1. Field of the Invention
The present invention is directed to a chromium compound-containing catalyst for polymerizing alpha-olefins, also known as 1-olefins, and to a method of polymerizing such olefins. More particularly, the invention is directed to a method of producing polymers of ethylene or copolymers of ethylene and at least one C.sub.3 -C.sub.10 alpha-olefin, which are useful in the fabrication of products for blow molding applications, especially for the household and industrial containers (HIC) market, at lower temperatures than was heretofore possible.
2. Description of the Prior Art
Chromium compound-containing alpha-olefin polymerization catalysts, also known as Phillips catalysts, have been extensively described in the literature. They are formed by supporting chromium trioxide, or a compound calcinable thereto, on a refractory oxide support material, for example, silica, alumina, zirconia, thoria or silica-alumina, and heating the oxide support material in a non-reducing atmosphere, preferably an oxidizing atmosphere, to produce an active polymerization catalyst. The produced catalyst is used to polymerize 1-olefins in a suitable process, such as the so-called "solution form" or "particle form" process. In the "solution form" process, the monomeric 1-olefin, which is normally ethylene or a mixture of ethylene with up to about 40 wt. % of other 1-olefins, is contacted with a suspension of the catalyst in a liquid hydrocarbon which is a solvent for the polymer at the polymerization temperature employed. In the "particle form" process, the monomer 1-olefin is contacted with a suspension or a fluidized bed of the catalyst particles in a fluid medium under conditions such that the polymeric 1-olefin forms as solid particles suspended in or fluidized in the fluid medium. The fluid medium can be, for example, a liquid hydrocarbon or a gas. Examples of suitable liquid hydrocarbons are isobutane and n-pentane. Examples of suitable gases are nitrogen or argon mixed with the gaseous monomer, or the undiluted gaseous monomer. Processes of this type are described in, for example, U.K. published patent specification Nos. 790,195; 704,641; 853,414; 886,784 and 899,156. It is also known to modify such catalysts with a titanium compound, for example, to render the catalyst capable of producing polyolefins having increased melt index (i.e., lower average molecular weight) or to increase the stress crack resistance of the produced polyolefin. Catalysts of this type are described in, for example, U.S. Pat. No. 3,622,521 to Hogan et al and U.K. published patent specification Nos. 1,334,662 and 1,326,167.
U.S. Pat. No. 3,351,623 to Walker et al discloses a catalyst for polymerizing ethylene at a temperature in the range of 275.degree. to 335.degree. F., i.e., under solution form process conditions, the catalyst being one which forms on mixing (1) an oxide component selected from the group consisting of silica; mixtures of silica and alumina containing up to 25 wt. % alumina; and mixtures of chromium oxide and at least one material selected from the group consisting of silica, alumina, zirconia and thoria, at least part of the chromium oxide being in the hexavalent state at the initial contacting of the monomer therewith, (2) an organo metallic component of formula R.sub.x M where R is selected from the group consisting of hydrogen and alkyl, aryl, cycloalkyl, alkoxy radicals and combinations of these radicals containing up to 12 carbon atoms, at least one R group being a hydrocarbon radical; M is selected from the group consisting of aluminum, gallium, indium, thallium, zinc, boron, lead, lithium, zirconia, cobalt, magnesium and tin; and x is an integer equal to the valence of M, and (3) a vanadium component selected from the group consisting of vanadium chelates and vanadyl chelates. Walker et al seek to produce a catalyst that permits the use of high polymerization temperatures to obtain relatively low melt index polymers.
Downs, U.S. Pat. No. 4,368,302, discloses a modified alpha-olefin catalyst composition used in preparing polymers of ethylene or copolymers of ethylene and higher alpha-olefins, having high average molecular weight and a relatively broad molecular weight distribution, as evidenced by relatively high values of melt flow ratio (MFR), referred to by Downs as melt index ratio (MIR). The catalyst composition of Downs is prepared by supporting chromium oxide on a refractory oxide support material, adding at least one tetravalent titanium compound and heating to activate the product. The monomer is contacted with the active polymerization catalyst in the presence of one or more organometallic compounds, e.g., dibutylmagnesium or triethylaluminum. The amount of the organometallic compounds is 0.1-100% by weight (wt%), preferably 1 to 10 wt %. The refractory oxide support has a mean particle diameter of about 20-150 microns (.mu.).
U.K. patent application No. 2,023,153 to Eve discloses an ethylene polymerization catalyst comprising: (A) a heat activated supported chromium oxide catalyst, and (B) a catalyst modifier comprising a magnesium compound, such as a dialkyl magnesium, preferably dibutyl magnesium, and a commercially available product believed to comprise a mixture of dibutyl magnesium, dialkyl magnesium and butyl-alkyl magnesium. The polymerization process, conducted in the presence of this catalyst, produces polyethylene or ethylene copolymers having broad molecular weight distribution and low melt index. The quantity of the catalyst modifier is such that the atomic ratio of magnesium to chromium in the modified catalyst system is 0.01:1 to 100:1, preferably, 0.01:1 to 10:1.
Stricklen et al., U.S. Pat. No. 4,374,234, discloses a silica-supported chromium catalyst to which is added up to 5 ppm of an aluminumalkyl or dihydrocarbomagnesium compound, e.g., dibutylmagnesium, in order to reduce the induction period and increase catalyst activity. The addition of the aluminum or the magnesium compounds has only a modest effect on polymer properties.
When polymers of ethylene or copolymers of ethylene and of one or more C.sub.3 -C.sub.10 alpha-olefin useful for the blow-molding applications, e.g., for the HIC market, are produced using the above-identified prior art catalysts, e.g., in a slurry reactor, the reaction must either be conducted at relatively high temperatures of about 108.degree.-110.degree. C., approaching melting or sintering temperatures of the product resin, or with a relatively high level of oxygen intentionally added to the reactor (oxygen add-back) to produce a resin having the desired high load melt index (HLMI).
The operation of some reactors, such as slurry reactors, at the high temperature requires very elaborate control apparatus to prevent fusion and agglomeration of the resin particles, particularly the small size resin particles (fines), e.g., particles of the size less than 74 microns (.mu.) in the reactor, e.g., on the reactor walls, and in the filter separating solid particles from the solvent. Since a typical resin produced with such a catalyst begins to melt and agglomerate at about 115.degree. C., even minute temperature excursions above the reactor operating temperature can cause severe fines agglomeration, referred to in the art as fouling, in the reactor and auxiliary process equipment.
Additionally, as is known to those skilled in the art, when fluid bed alpha-olefin polymerization reactor systems are utilized, a recirculation compressor is used to recycle the fluidizing gas and unreacted monomers, if any, to the fluid bed reactor. The recycled fluidizing gas also contains at least some entrained polymer particles. The system also utilizes heat exchangers, usually located downstream of the compressor, to remove substantial amounts of exothermic heat generated during the reaction (e.g., See Goeke et al, U.S. Pat. No. 4,302,565). The compressor inherently increases the temperature of the gas stream compressed therein. Since the temperature gradient between the fluid bed reactor operating temperature and the melting temperature of the resin is very small (e.g., about 7.degree.-9.degree. C.), even a small elevation of the temperature of the fluidizing gas may cause fusion and agglomeration of the polymer particles in the compressor. If the temperature of the recycle gas is increased to the level wherein the fusion of the polymer particles takes place, the polymer particles will also tend to fuse and agglomerate in the heat exchangers and on the distribution plate of the reactor utilized to provide a uniform fluidized bed of particles therein.
In such fluid bed polymerization systems, the danger of polymer particles' fusion is exacerbated by the presence of catalyst particles in the polymer particles. As is known to those skilled in the art, the product polymer particles, also referred to herein as resin particles, produced in the polymerization reactor system, contain minute amounts of the catalyst particles which are not removed therefrom because the content thereof in the resin is so small as to render the removal of the catalyst particles unnecessary. The catalyst is inactivated when the resin is removed from the reactor. However, the resin in the reactor system contains catalytically-active catalyst particles. These catalytically-active catalyst particles continue to promote the alpha-olefin polymerization reaction which is exothermic in nature, thereby producing an additional amount of heat. This heat, together with the inherent increase of the recycle gas temperature in the recycle compressor, combines to dangerously decrease the safety temperature margin between the reactor operating temperature and the resin fusion temperature. The polymer particles most susceptible to fusion are polymer fines. If fusion and agglomeration of polymer particles takes place in the recycle compressor, heat exchangers, distribution plate and/or any other parts of the polymerization system, the process efficiency is substantially decreased, and, ultimately, the fluidized bed system may have to be shut down to remove the fused polymer particles.
As mentioned above, the HLMI of the resin may also be increased by increasing the amount of oxygen added to the reactor, such as a slurry reactor, e.g., see Dalig et al. KHIMIYA I TEKNOLOGIYA POLYMEROV, Vol. 23, No. 4 (1961), Ermakov et al, "Transfer Processes During Polymerization of Ethylene on a Chromium Oxide Catalyst. II. The Role of Impurities In Transfer Reactions", KINETICS AND CATALYSIS (USSR), Vol 10, No. 333 (1969). However, the increased oxygen content in the reactor may promote the formation of polymer fines, especially in a fluid bed reactor, which are most likely to fuse in such a reactor system. The alpha-olefin fluidized bed reactors, unlike fluidized bed reactors used in different chemical processes, e.g., fluid catalytic cracking, do not usually utilizes fines removing equipment, such as cyclones or filters, because it is feared that such equipment may provide additional sites for fines to fuse and agglomerate. Thus, any polymer fines produced in the polymerization system tend to remain in the reactor loop. Accordingly, the use of relatively high amounts of oxygen to obtain resins of a desired HLMI may also lead to the fouling of the process equipment, such as compressor and heat exchangers, and, ultimately, to the shut-down of the reactor system.
Accordingly, it is a primary object of this invention to provide an improved polymerization catalyst which produces polymers of HLMI suitable for the blow molding applications at substantially lower operating temperatures than was heretofore possible with other catalysts utilized in polymerization reactions at comparable conditions.
It is an additional object of the present invention to provide an alpha-olefin polymerization process which produces polymers of ethylene and/or C.sub.3 -C.sub.10 alpha-olefins, having such an HLMI that renders them suitable for the blow molding applications at lower temperatures than was heretofore thought possible, and with reduced polymer fines formation.
Additional objects of the invention will become apparent to those skilled in the art from the following specification and the attached claims.